1. Field of the Invention
The present invention generally relates to a restart up circuit, in particular, to a restart up circuit for a VCO (voltage-controlled oscillator) of a PLL (phase-locked loop), and a method to restart up the VCO of the PLL.
2. Description of Related Art
In order to avoid the VCO of the PLL being in a stable mode, the operation of the PLL typical needs a power down signal to start up the VCO initially.
However, if the VCO starts up fail and is still in a stable mode, the power down signal needs to be sent to the PLL again and forces the VCO to restart up manually. Using the power down signal to restart up the VCO may influence the PLL loop stability.